Add versioned, ABI-tagged inline namespace and namespace macros (#3590)

* Add versioned inline namespace

Add a versioned inline namespace to prevent ABI issues when linking code
using multiple library versions.

* Add namespace macros

* Encode ABI information in inline namespace

Add _diag suffix to inline namespace if JSON_DIAGNOSTICS is enabled, and
_ldvcmp suffix if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON is enabled.

* Move ABI-affecting macros into abi_macros.hpp

* Move std_fs namespace definition into std_fs.hpp

* Remove std_fs namespace from unit test

* Format more files in tests directory

* Add unit tests

* Update documentation

* Fix GDB pretty printer

* fixup! Add namespace macros

* Derive ABI prefix from NLOHMANN_JSON_VERSION_*

include/nlohmann/detail/input/binary_reader.hpp

@@ -32,8 +32,7 @@
 #include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
 
@@ -2969,5 +2968,6 @@ class binary_reader
     /// the SAX parser
     json_sax_t* sax = nullptr;
 };
+
 }  // namespace detail
-}  // namespace nlohmann
+NLOHMANN_JSON_NAMESPACE_END